The present application relates generally to non-contact capacitive displacement sensors, and more specifically to a non-contact capacitive displacement sensor capable of accurately measuring a distance between the sensor and a shaped target.
Non-contact capacitive displacement sensors are known that may be employed to measure small distances between a sensor element and a target element by detecting variations in the capacitance between the sensor and the target. A conventional non-contact capacitive displacement sensor typically comprises a probe device including a sensor element, a guard element, and a ground element. The guard element is configured to surround the sensor element, thereby making the sensor less sensitive to capacitance that might develop between the sensor and other portions of the probe. The ground element is configured to surround the guard element. Specifically, the ground element constitutes a part of the structure of the probe. For example, the ground element may be configured as a ring providing a surface for use in mounting the probe. It is noted that the guard and/or the ground ring may be omitted from some probe configurations. The sensor element, the guard element, and the ground element of the conventional probe device typically comprise respective machined metallic parts that are bonded together in a manufacturing process to form the overall probe structure.
In a typical mode of operation, the sensor element of the conventional probe device forms one plate of a capacitor, the target element forms the other plate of the capacitor, and the distance between the probe and the target is determined by detecting the variation in the capacitance between the respective conductive plates. The detected capacitance varies with the distance between the probe and the target, and this variation in capacitance provides a measurement signal that may subsequently be processed to produce an output signal indicative of the distance or a change in the distance.
However, the conventional non-contact capacitive displacement sensor described above has several drawbacks. For example, because the various elements of the conventional probe device typically comprise individual machined parts, the process for manufacturing the probe can be highly labor intensive, which can cause an increase in the overall cost of the sensor.
In addition, using the conventional non-contact capacitive displacement sensor to obtain distance measurements can be problematic when measuring the distance between the sensor and a shaped target, i.e., a target element having a shape that does not approximate a flat conductive plate. For example, the target element may be a disk, and it may be desirable to use the conventional probe device to measure a distance between the probe and an edge of the disk. In this example, the edge of the disk is not flat, but instead follows the circular contour of the disk. As a result, the combination of the sensor element and the target element (i.e., the disk edge) does not form a parallel-plate capacitor, and conventional methods of determining the capacitance between two parallel plates cannot be used for accurately measuring the distance between the probe and the shaped target.
It would therefore be desirable to have an improved non-contact capacitive displacement sensor. Such a capacitive displacement sensor would be capable of accurately measuring small distances between the sensor and shaped targets. It would also be desirable to have a non-contact capacitive displacement sensor that is easier and less costly to manufacture.